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Only 10% of the cells in your body are human

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There are about ten times more bacteria cells in your body than there are cells with human DNA. According to Dr. Cynthia Sears at Johns Hopkins, your body contains something like 100 trillion bacteria from 500 to 1000 different species, compared to only about 10 trillion of your own cells.

Don't look now, but we're outnumbered. In fact, there are as many bacteria cells just on the surface of your body as human cell in your body altogether.

The average human is more microbe than mammal, a veritable super-organism comprising 10 times as many microbial cells as human cells. With so many microscopic hangers-on you can afford to shed a few. But you would be in big trouble without any at all. In fact, you wouldn't be human, a paradox that scientists are trying hard to get their heads around.

Until recently, our knowledge has been limited, not least because it was only possible to identify those microbes that could be cultivated in the lab - a mere 1 to 2 per cent of the full quota. Very few studies exist on the bugs that are up your nose, let alone further along the respiratory tract, where there are several different environments, each harboring different microbes. We are pretty ignorant about the eye, too. And when it comes to the female urethra, forget it: most of what we know dates back to the 1970s and primarily relates to women's high susceptibility to urinary tract infections.

In May of 2007, the US National Institutes of Health approved a five-year plan to investigate the human microbiome - the entire microbial content of the human body. The next few years will bring a massive leap in our understanding, but already smaller-scale projects, investigating the microbes of the gut and mouth, for example, are beginning to throw up some intriguing insights.

No two individuals have the same microbial complement, and it even changes over time for each person - but they all share a bacterial core or scaffold. Given that skin is the boundary between you and the outside world, it is hardly surprising that its microbial community is so variable. The balance is extremely sensitive to environmental fluctuations, changing each time you have a shower or even use a new brand of soap. Such differences are there right from the start, with babies delivered by Caesarean section having a different profile of microbes on their skin to those born naturally and therefore exposed to the microbes in their mother's birth canal. We don't know whether these differences are long-lasting, only that by six weeks the complement of microbes living on the skin of infants resembles that found on adults.

Skin bacteria can cause serious infections if they get into your blood, as they sometimes do via hospital catheters, says Mike Wilson, a microbiologist at the Eastman Dental Institute, part of University College London, and author of Microbial Inhabitants of Humans. Some regular skin-dwellers also do damage in situ, causing a multitude of skin complaints, from athlete's foot and impetigo to dandruff.

The rapid flow of partly digested food through your small intestine makes it a poor home for microbes, since the bugs are constantly being washed through. In the colon, however, things slow down and microbial concentrations soar. Thousands of different types of microbe inhabit the gut, an estimated kilogram's worth in the average adult. Without them we would not be able to digest certain foods, metabolise drugs, detoxify noxious compounds or make essential vitamins.

There are two main phyla of bacteria in the colon, the Bacteroidetes and the Firmicutes, as well as Archaea (evolutionarily ancient, single-celled organisms that consume hydrogen and generate methane). It has long been known that gut microflora changes with age, diet and other factors. For example, there is a significant difference in the ratio of microbial groups between breastfed and bottle-fed babies. In breastfed babies the community is qualitatively less pathogenic. Although gut microbes in infants all converge towards a more adult profile by the age of 2, any pathogen is potentially more dangerous before this time because the baby's immune system is still developing. So breast is best from the point of view of the indigenous microbiota.

If we are not using our mouths to chew or swallow food, we're talking, gargling or brushing our teeth. Even so, some bugs do manage to hang on in there. For the most part they do no harm, but under certain circumstances they can cause problems.

The worst of these is the gum disease periodontitis, the most prevalent chronic infectious disease in humans. It begins when normally benign mouth bacteria are allowed to accumulate, forming a sticky layer of plaque at the interface between teeth and gums. This creates an environment in which anaerobic bacteria can thrive, producing enzymes that degrade the surrounding tissues, triggering an inflammatory response and eventually eroding the alveolar bone in which the teeth are embedded. Caries, or cavities, have received more airtime, though. You get them when Streptococcus mutans, a component of plaque, ferments sugary foods to produce lactic acid. This not only erodes the tooth's enamel surface, it also allows S. mutans to proliferate. The bug starts outgrowing the other bugs, many of which can't survive in low pH.

Caries could soon be a thing of the past, though. The University of Florida College of Dentistry has come up with an ingenious strategy for preventing them. It involves a genetically modified strain of S. mutans that also thrives on sugars but, instead of producing lactic acid, secretes an antibiotic that kills all other strains of the bacterium. The treatment, which is now being tested for safety in clinical trials, aims to provide lifelong protection with just a single, 5-minute application.

"Not only does the Earth contain more bacterial organisms than all others combined (scarcely surprising, given their minimal size and mass); not only do bacteria live in more places and work in a greater variety of metabolic ways; not only did bacteria alone constitute the first half of life's history, with no slackening in diversity thereafter; but also, and most surprisingly, total bacterial biomass (even at such minimal weight per cell) may exceed all the rest of life combined, even forest trees, once we include the subterranean populations as well."

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